So far, the maximum output of a motor used in a machine such as a shaper, a press, a cutter, a roller, etc. is determined according to the maximum work load of the machine driven by the motor. Therefore, when a machine is idling or working with a light load, the motor still revolves nearly at its high output, which results in the waste of electric power.
This invention reduces as much as possible the waste of electric power used in the motor driving such a machine as mentioned above.